TOKYO (BLOOMBERG) - Japan was one of first countries outside of China hit by the coronavirus and now, it is one of the least-affected among developed nations. That's puzzling health experts.

Unlike China's draconian isolation measures, the mass quarantine in much of Europe and big United States cities ordering people to shelter in place, Japan had imposed no lockdown. While there have been disruptions caused by school closures, life continues as normal for much of the population. Tokyo rush hour trains are still packed and restaurants remain open.

The looming question is whether Japan dodged a bullet or is about to be hit. The government contends it has been aggressive in identifying clusters and containing the spread, which makes its overall and per capita number for infections among the lowest among developed economies. Critics argue Japan has been lax in testing, perhaps looking to keep the infection numbers low, as it's set to host the Olympics in Tokyo in July.

Japan's initial slow response to the virus, its handling of the Diamond Princess cruise ship - where about one in five people aboard became infected while it was quarantined in Yokohama - and the decision not to initially block travel from China left the nation open to criticism that it could become home to a "second Wuhan". Steps taken to contain the virus - such as shutting schools and calling off large events - now look tame in comparison to what others have done.

But as of Wednesday (March 18), Japan has only had a little more than 900 confirmed cases - excluding the cruise ship. The US, France and Germany were all above 7,000 cases and Italy was nearing 36,000. Neighbour South Korea, which tested aggressively amid a surge of confirmed infections from late February, was at about 8,500 cases but its new infections are now tapering off.

In Tokyo, among the world's most densely packed metropolitan areas, cases made up 0.0008 per cent of the population. The northern main island of Hokkaido, the skiing destination that was Japan's worst-hit area, is already lifting a state of emergency, as new cases have slowed.

Professor Kenji Shibuya of King's College London and a former chief of health policy at the World Health Organisation sees two possibilities: that Japan has contained the spread by focusing on outbreak clusters, or that there are outbreaks yet to be found.

"Both are reasonable, but my guess is that Japan is about to see the explosion and will inevitably shift from containment to delay-the-peak phase very soon," he said. "The number of tests is increasing, but not enough."

Japan's proximity to China may have helped in raising the alarm when the disease was in a more controllable phase. In late January, shortly after Japan's first infection of a person who had not been to China, hand sanitisers starting popping up in offices and stores, mask sales spiked and people began to accept some basic steps to protect public health. This may have also helped Japan flatten the curve for infections.

"Japan has been fortunate that only a small number of cases of SARS-CoV-2 were brought into the country, and they seem to have remained concentrated in finite areas, easy to control," said Mr Laurie Garrett, an American global health writer, referring to the technical name of the coronavirus.

Despite the infectiousness of the virus, a March 9 report by a government-appointed panel said that about 80 per cent of the cases identified in Japan didn't pass on the infection. But there's little consensus over why and scepticism over whether the same government that was issued a rare rebuke by US health authorities for letting the Diamond Princess outbreak get out of hand is getting it right on coronavirus.

"Many infection clusters have been identified at a comparatively early stage," the panel said in a report this month. Prime Minister Shinzo Abe cited those findings when he said last Saturday that Japan didn't yet need to declare a state of emergency.

Japan may have some built-in advantages, such as a culture where handshakes and hugs are less common than in other Group of Seven countries. It also has rates of hand-washing above those in Europe.

Cases of seasonal flu have been declining for seven straight weeks, just as the coronavirus was spreading, indicating the Japanese may have taken to heart the need to adopt some basic steps to stem infectious diseases. Tokyo Metropolitan Infectious Disease Surveillance Centre data shows that influenza cases this year are well below normal levels, with nationwide cases at the lowest, according to data going back to 2004.

Japan has ramped up its capacity but has tested only around 5 per cent the number of people as neighbouring South Korea has, despite a larger population. But the situation in Italy, which tested extensively only to see hospitals overwhelmed, has also given some pause.

"Italy's mortality rate is almost triple Japan's," said Prof Yoko Tsukamoto, a professor of infection control at the Health Sciences University of Hokkaido. "Part of the reason is if you get tested, you get quarantined, so it means that they don't have enough beds for relatively non-severe patients."

Japan has tested more than 15,000 people as of Wednesday, and despite discouraging checks on those who don't have symptoms or contact with a carrier, the infection rate lies at 5.6 per cent. That compares to around 3 per cent in South Korea, but 18 per cent in Italy.

"We don't see a need to use all of our testing capacity, just because we have it," Health Ministry official Yasuyuki Sahara said at a briefing on Tuesday. "Neither do we think it's necessary to test people just because they're worried."

Should Japan see a jump, it may be better suited than many peers to handle the surge. It has about 13 hospital beds per 1,000 people, the highest among G-7 nations and more than triple the rate for Italy, the US, Britain and Canada, according to World Bank data.

Even if Japan may not be counting all those infected, hospitals aren't being stretched thin and there has been no spike in pneumonia cases, health officials said. While the prime minister has stepped up border controls, his government is mulling whether to ease restraint at home, such as allowing students back to class when the new school year starts in April.

"We will do all that is possible to end the coronavirus outbreak," Prime Minister Abe told his ruling party this week.